The present invention relates generally to satellite/laser positioning systems, and more particularly to accurately determining height coordinates in a satellite/laser positioning system.
There are various types of positioning systems for determining the position of an object. For example, in a global navigation satellite system (GNSS) a navigation receiver receives and processes radio signals transmitted by satellites. Examples of such GNSS systems are the Global Positioning System (GPS) of the United States, the Global Navigation Satellite System (GLONASS) of Russia, and the planned Galileo system of Europe. Generally, the term GPS will be used herein, but it should be recognized that the discussion herein is equally applicable to any type of GNSS.
A GPS receiver measures the time delay of received satellite signals relative to a local reference clock. These measurements enable the receiver to determine the so-called pseudo-ranges between the receiver and the satellites. If the number of satellites is large enough, then the measured pseudo-ranges can be processed to determine the user location and time. The accuracy of the location determination may be increased through the use of various techniques. One such technique is differential navigation (DN) in which the task of finding the user position, also called the rover, is performed relative to a base station at a known location. The base station has a navigation receiver which receives and processes the signals of the satellites to generate measurements. These signal measurements are transmitted to the rover via a communication channel (e.g., wireless). The rover uses these measurements received from the base, along with its own measurements taken with its own navigation receiver, in order to determine its location precisely. The location determination is improved in the differential navigation mode because the rover is able to use the base station measurements in order to compensate for errors in the rover measurements.
The location determination accuracy of differential navigation may be improved further by supplementing the pseudo-range measurements with measurements of the phases of the satellite carrier signals. If the carrier phase of the signal received from a satellite in the base receiver is measured and compared to the carrier phase of the same satellite measured in the rover receiver, measurement accuracy may be obtained to within several percent of the carrier's wavelength.
The above described general scheme of computations is well known in the art and is described in further detail, for example, in, Bradford W. Parkinson and James J. Spilker Jr., Global Positioning Theory and Applications, Volume 163 of Progress In Astronautics and Aeronautics, published by the American Institute of Aeronautics and Astronautics, Inc, Washington D.C., 1996. A real-time-kinematic (RTK) GPS system, which utilizes satellite carrier phase in combination with differential navigation techniques is described in U.S. Pat. No. 6,268,824, which is incorporated herein by reference.
The above described navigation techniques result in highly accurate horizontal position measurements. However, one known deficiency in GPS location techniques is a lack of accuracy in vertical position measurements. As such, determining the height of a GPS receiver cannot be determined with the same accuracy as that for the horizontal measurements.
One technique for increasing the accuracy of height calculations is to supplement the GPS calculations with another system. For example, U.S. Patent Application Publication No. US2004/0125365 A1, entitled Working Position Measuring System, which is hereby incorporated by reference in its entirety, discloses a system that accurately determines the vertical angle (i.e., elevation angle) from an appropriately equipped rotating laser transmitter to a laser receiver. The rotating laser system generally includes a rotating laser at a fixed location, with a photodetector at the target location. The photodetector periodically detects the rotating laser beam and generates a signal based upon receipt of the laser (i.e., when the laser beam strikes a photocell of the detector). In an advantageous embodiment, the transmitted laser beam comprises fan shaped beams in the shape of the letter N. The signal may be processed using various techniques in order to provide additional positioning/geometric information, such as the vertical angle between the photodetector and the laser transmitter.
The above described rotating laser system itself only measures the vertical angle between the photodetector and the laser transmitter, and does not measure the height of the target. Using well known geometry (as will be discussed in further detail below), given the vertical angle between the photodetector and the laser transmitter, the relative height of the photodetector and the laser transmitter can be determined if the horizontal separation distance between the laser transmitter and the photodetector is known. Since the absolute height of the laser transmitter is known, the absolute height of the target can be determined once the relative height of the photodetector and the laser transmitter is calculated.
The horizontal separation distance between the laser transmitter and the photodetector may be determined using GPS techniques. In fact, the two systems complement each other. As discussed above, GPS techniques can provide highly accurate horizontal measurements, but less accurate vertical measurements. On the other hand, the rotating laser system can provide highly accurate vertical angles, but can only provide accurate height measurements if the vertical angle is supplemented with sufficiently accurate horizontal measurements. As such, an advantageous combination of the two systems provides highly accurate positioning in both horizontal and vertical measurements. Such a combined system is described in further detail in the above referenced U.S. Patent Application Publication No. US2004/0125365 A1. As shown in the referenced patent application Publication, such a system may be used in combination with a survey pole for use in connection with accurate survey applications, and with an earthmoving machine for use in connection with accurate construction applications.
One problem with the above described satellite/laser positioning system arises when the laser signal becomes unavailable for some reason. For example, in a construction application, machinery or structures may come between the rotating laser transmitter and the laser receiver of the rover. During obstruction of the line of sight between the laser transmitter and the laser receiver of the rover, there is a disruption in the available height information. This results in a reduced accuracy of the position determination.